1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. The present invention is suitably applied to image forming apparatuses such as digital copying machines, multi function printers and laser beam printers (LBP) using an electrophotography process.
2. Description of the Related Art
Various optical scanning apparatuses that use, as an optical deflector for reflecting and deflecting a light beam, one that moves or swings back and forth have been conventionally proposed (see for example, Japanese Patent Application Laid-Open No. 2004-191416).
Optical scanning apparatuses using an optical deflector that swings back and forth are advantages, for example, in that the size of the optical deflector can be made much smaller and electric power consumption is low, as compared to optical scanning apparatuses that use, as an optical deflector, a multi-face rotating mirror such as a polygon mirror. In particular, optical deflectors made of a Si single crystal manufactured using a semiconductor process are advantageous in that they are theoretically free from metal fatigue and have excellent durability.
In the technology disclosed in Japanese Patent Application Laid-Open No. 2004-191416, the width, with respect to the main scanning direction, of a light beam incident on an optical deflector that oscillates sinusoidally is designed to be longer than the width, with respect to the main scanning direction, of the optical deflector, whereby the deflection surface of the optical deflector is used efficiently. With this feature, the spot diameter is intended to be made smaller, and an optical scanning apparatus in which adjustment of the position of an incident optical system can be eliminated is realized.
The deflecting surface of the optical deflector such as the deflection means that oscillates sinusoidally is a single surface, unlike with rotary multi-face mirrors that are widely used.
On the other hand, in multi-face rotating mirrors that are generally used, the plurality of deflection surfaces are arranged to be perpendicular to the main scanning direction. However, if an optical face tangle error occurs in a deflection surface with respect to the sub scanning direction due to, for example, a manufacturing error, the spot on a photosensitive drum is displaced in the sub scanning direction.
To correct the displacement, in current imaging optical systems, an anamorphic optical system called an optical face tangle error correction optical system in which the deflection surface and the surface to be scanned are arranged to be conjugate with each other is used.
In the optical face tangle error correction optical system, the deflection surface and the surface to be scanned are arranged in a conjugate relationship in the sub scanning cross section. Accordingly, if the position of the imaging lens that constitutes the imaging optical system is shifted toward the deflection surface with the intention of reducing its size, the imaging magnification in the sub scanning cross section becomes high, which leads to the problem that the sensitivity of the positioning of the imaging optical system is likely to become high.
In the case where the imaging lens that constitutes the imaging optical system is made of a plastic material with which the lens can be manufactured easily, focus errors in the sub scanning direction can occur due to changes in the refractive index of the lens material caused by increases in the temperature of the apparatus.
The defocus amount increases with an increase in the imaging magnification in the sub scanning cross section. Therefore, in the case where the imaging lens is made of a plastic material, it is very difficult to make the position of the imaging lens closer to the deflection surface to reduce the size.